HDFS-4699. Additional conditions for avoiding unnecessary DataNode.checkDiskError calls, as part of TestPipelinesFailover#testPipelineRecoveryStress fails sporadically. Contributed by Chris Nauroth

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-1@1469843 13f79535-47bb-0310-9956-ffa450edef68